Untersuchungen über Allylgruppenwanderungen in aliphatischen Carbenium‐Ionen

Investigations on the Migratory Aptitude of Allyl Groups in Aliphatic Carbenium‐Ions The acetolysis (80°) of the 4‐bromobenzenesulfonates given in Scheme 6 were investigated in regard to determine type allyl/methyl migratory aptitudes in the secondary carbenium ion a (Scheme 24) . In all cases olefins (about 80%) and acetates (about 20%) were formed which can be derived from the rearranged tertiary carbenium ions b (being formed by allyl group migration) and c (being formed by methyl group migration). Olefin A and acetate H , originated in carbenium ion a, occurred in the acetolysis mixture only in minor amounts (<2%). By acetolysis of [l4C]‐20, isolation of [ 14 C]‐4,5‐dimethyl‐l, 3‐hexadiene ([ 14 C]‐45), and degradation of this diene (Scheme 16) it could be shown (4 Scheme 15) that the ions b and c (Scheme 24, R 1 R 4 H) are not interconverted by a [1,2]‐hydride shift (extent < 1%). Since olefin D arises by proton loss from ion b as well as from ion c , [ 14 C]‐4,5‐dimethyl‐l,4‐hexadiene ([ 14 C]44 D, R 1 R 4 H) was also degraded ( cf. Scheme 15 and Scheme 17 ). It was found that [ 14 C]‐ 44 contained 48% of the label in the methyl group at C( 4 ) and 52% in the methyl groups at C( 5 ), i.e. 48% of 44 is formed via the allyl migration path and 52% via the methyl migration path. In addition, acetolysis of d3‐20 and product analysis showed, that the d3‐ally1 moiety migrates as expected only in a [1,2]‐fashion. Product analysis of the acetolysis mixtures of erythro ‐ and threo ‐ 24 ( cf. Scheme 19 and Tables 4 and 5 ) revealed that carbenium ion a must exist as an intimate ion pair (with the 4‐bromobenzenesulfonyloxy‐ion) which has lost its configuration at C( 1 ) only partially. This is indicated by reversed ratios (1: 11 and 10: 1, resp.) in the formation of erythro ‐ and threo ‐2,3,4‐trimethyl‐l, 5‐hexadiene ( erythro ‐ and threo ‐ 77 ) arising from ion b ( Scheme 24, R 1 R 3 H, R 4 CH,). The acetolysis of 1,2,2,4‐tetramethyl‐4‐pentenyl4‐bromobenzenesulfonate ( 23 ) was not studied in detail, but the appearance of a seventh product in the olefin part cannot be explained by the genesis paths in Scheme 24. Thus, it may be concluded that in this case a third tertiary carbenium ion d 3 (Scheme 21) is produced by cyclization of a 3 . Cyclizations of this type are known to occur in carbenium ions bearing β‐substituted allyl groups (see Scheme 22 ). The kinetic data of the acetolysis of all 4‐bromobenzenesulfonates ( Table 6 ) are in accord with a rate determining ionization step leading to a since all activation enthalpies resp. entropies are within 25.5 L± 0.6 kcal/mol resp. −0.2 ± 1.7 e.u. The migratory aptitudes given in Table 7 show, that allyl groups migrate only slightly easier than methyl groups in ion a . This is in strong contrast to allyl substituted methylcyclohexadienyl cations (generated in the acid catalyzed dienone/phenol and dienol/benzene rearrange‐ment) which undergo exclusively [1,2]‐ally1 migrations ( Schemes 3‐5 ).